Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (“NOx”). Emission control systems, including exhaust gas catalysts, are widely utilized to reduce the amount of these pollutants emitted to atmosphere. A commonly used catalyst for gasoline engine applications is a three-way catalyst (TWC). TWCs perform three main functions: (1) oxidation of carbon monoxide (CO); (2) oxidation of unburnt hydrocarbons; and (3) reduction of NOx to N2.
TWC catalysts require careful engine management techniques to ensure that the engine operates at or close to stoichiometric conditions (air/fuel ratio, λ=1). For technical reasons, however, it is necessary for engines to operate on either side of λ=1 at various stages during an operating cycle. When the engine is running rich, for example during acceleration, the overall exhaust gas composition is reducing in nature, and it is more difficult to carry out oxidation reactions on the catalyst surface. For this reason, TWCs have been developed to incorporate a component which stores oxygen during leaner periods of the operating cycle, and releases oxygen during richer periods of the operating cycle, thus extending the effective operating window. For such purposes, ceria-based (e.g., ceria-zirconia mixed oxides) materials are used in the vast majority of current commercial TWCs as oxygen storage components (OSC).
Aggregation of precious metal particles is known as one factor that causes deterioration of TWC catalysts used in the purification of gasoline engine exhaust gas. In particular, rhodium is known to increase aggregation rate by transforming to rhodium oxide in high-temperature/oxidizing atmospheres, and aggregation causes the surface area of catalyst activation points to become smaller resulting in a deterioration in catalyst performance.
Despite advances in TWC technology, there remains a need for improved catalytic converters for certain engine platforms that produce high conversion rates with improved thermal stability. This invention solves these needs amongst others.